Aluminum metal is prepared on an industrial scale by the Hall-Heroult aluminum electrolysis process.
In a typical arrangement an electrolysis cell is lined with carbon, which acts as the cathode. Iron or steel bars are embedded in the cathode lining to provide a path for current flow. The anodes are also of carbon and are gradually fed into the top of the cell because the anodes are continually consumed during electrolysis. Several cells may be connected in series.
For aluminum, the electrolyte used is typically cryolite (Na.sub.3 AlF.sub.6) containing, when the Al.sub.2 O.sub.3 is added by point feeders, 2 to 4% dissolved Al.sub.2 O.sub.3. Other additives, such as CaF.sub.2 (up to 6%) and AlF.sub.3 (up to 12%), are added to obtain desirable electrochemical properties. The Hall-Heroult cell operates at temperatures of approximately 960.degree. C. (1760.degree. F.).
At the cathode of the aluminum cell, aluminum is reduced from an ionic state to a metallic state, through a series of complex reactions. The metallic (reduced) molten aluminum forms a molten pool in the bottom of the cell. Periodically, an amount of metal is drained or siphoned from the molten pool of aluminum metal at the bottom of the cell.
At the anode, oxygen is oxidized from its ionic state to oxygen gas. The oxygen gas in turn reacts with the carbon anode to form carbon dioxide gas, thereby gradually consuming the anode material. Two types of anodes are in use: prebaked and self-baking. Prebaked anodes are individual carbon blocks that are replaced one after another as they are consumed. Self-baking anodes, are made up of a carbon paste which is fed into the cell from above. As the anode descends in the cell it hardens and new carbon paste is fed continually into the top of the cell.
If impurities in the aluminum oxide raw material are carefully controlled, aluminum with a purity of 99.7% or higher may be produced.
The following references provide a general discussion of various aspects of fusion electrolysis extraction of aluminum, particularly the design and operation of electrolysis cells.
(1) Winnacker/Kuechler; Chemische Technologie (Chemical Technology), Vol. 4, Fourth Edition, Carl Hanser Verlag Munich, 1986, Aluminum Chapter, pp. 252-282 PA0 (2) Grjotheim, K. and B. J. Welch: Aluminum Smelter Technology, Aluminium-Verlag, Duesseldorf, 1980 PA0 (3) Light Metals 1986, edited by R. E. Miller, Proceedings of the 115th AIME Annual Meeting, New Orleans, March 1986, pp. 343-347, The Metall. Soc. Inc., Warrendale, Pa., USA PA0 (4) Hall-Heroult Centennial, First Century of Aluminum Process Technology 1886-1986, edited by W. S. Peterson and R. E. Miller, presented at the 115th TMS Annual Meeting, New Orleans, March 1986, The Metall. Soc. Inc., Warrendale, Pa., USA PA0 (5) Wilkening, S.: Gewinnung von Aluminium durch Schmelzflusselektrolyse, Praxis der Naturwissenschaften (Extraction of Aluminum by Fusion Electrolysis, the Practice of the Natural Sciences) Chemie, Vol. 35, No. 3, 1986, pp. 21-25. PA0 (6) Lange, G. and G. Wilde, Large Aluminum Cells with Continuous Prebaked Anodes, Extractive Metallurgy of Aluminum, Vol. 2, edited by G. Gerrads, Interscience Publishers, New York, 1962, pp. 197-209 PA0 (7) Ginsberg, H. and S. Wilkening, Beitrag zur thermodynamischen und energetischen Betrachtung der Schmelzflusselektrolyse des Aluminiums (Contribution to the Thermodynamic and Energetic Analysis of the Fused-Mass Electrolysis of Aluminum), part II, Metall, Vol. 18 (1964), No. 9, pp. 908-918 PA0 (8) Winnacker, L. and L. Kuechler, Chemische Technologie (Chemical Technology), Vol. 6 of Metallurgie, pg. 194, Carl Hanser Verlag, Munich, 1973
To properly understand the process conditions of the present invention, the following theoretical relationships are set forth.
The energy theoretically required for the electrochemical reduction of Al.sub.2 O.sub.3 using a carbon anode is approximately 6.5 kWh/kg of aluminum. The technically most advanced electrolysis plants have achieved specific energy consumption rates of about 13 kWh/kg of aluminum, but this still signifies a relatively low efficiency of about 50%. The theoretical amount of current required to deposit 1 kg of aluminum is 2.980 kAh/kg of aluminum. For the current yields of 93 to 95%, attainable under the most advantageous operating conditions, 3.17 kAh/kg of aluminum are required on the average. The specific consumption of electrical energy results from the product of current consumption and cell voltage: EQU E=(C.times.U.sub.Z)/.eta.kWh/kg of Al
in which
C=2.98 kAh/kg of aluminum PA1 .eta.=current yield PA1 U.sub.z =cell voltage.
The cell voltage U.sub.Z is composed of the ohmic voltage drop of the cell IR.sub.Z and the polarization voltage U.sub.P : EQU U.sub.Z =I.times.R.sub.Z +U.sub.p EQU I=electrolysis current.
The ohmic resistance of the electrolysis cell R.sub.Z, which is responsible for the generation of heat, is distributed over the three essential regions of anode (R.sub.An), electrolyte or electrolysis bath (R.sub.Bath) and cathode (R.sub.Ca), in which the amounts of heat, E.sub.An =I.sup.2 .times.R.sub.An, E.sub.Bath =I.sup.2 .times.R.sub.Bath and E.sub.Ca =I.sup.2 .times.R.sub.Ca, are generated. The electrolysis cell is operated in a thermal equilibrium and it has always been the goal of those in the art to minimize energy consumption and heat losses for economical reasons.
On the assumption that the specific energy consumption at a current efficiency of 94% (3.17 kAh/kg of aluminum) is 13 kWh/kg of aluminum, a cell voltage U.sub.Z of 4.1 volt is obtained, for which the following division can be designated: EQU U.sub.An =0.4 V=I.times.R.sub.An EQU U.sub.Ca 0.4 V=I.times.R.sub.Ca ##EQU1##
If a polarization voltage U.sub.P of about 1.7 V is deducted from U.sub.Bath =3.3 V, approximately 1.6 V remains for the ohmic voltage drop (I.times.R.sub.bath =U.sub.Bath). For a given cross-sectional area of the electrodes, that is, the cathode and the anode, the voltage drops depend, of course, on the current density.
As is provided for pursuant to the invention, it is possible to double the anode and cathode surfaces in an electrolysis cell while keeping the current strength I (amperage) unchanged. In this case, the ohmic voltage drop in the electrolyte decreases by half, that is, from at least 1.6 V to 0.8 V. With that, 0.8 V.times.3.17 kAh/kg of aluminum=2.5 kWh/kg of aluminum less energy would be produced in the form of joulean heat, without any disadvantageous effect on the interpolar distance between the anode and the cathode or on the current yield. One of the results of the decrease in the energy consumption pointed out here leads, for example, to the above-mentioned total consumption of 10 to 11 kWh/kg of aluminum.
In comparison to the present state of the art, the following improvements, are achieved with the inventive electrolysis cell. For discussion purposes the inventive process is classified into three general areas: (1) the process overall; (2) the anode region; and (3) the cathode region.